1. Field of the Invention
This invention relates to a method and apparatus for controlling a plurality of temperature conditioning units such as a large number of heating and/or air conditioning units arranged to condition the air in a large enclosed area or multiple refrigeration units as may be found in a supermarket or a cold storage plant.
2. Prior Art
It is now common practice to heat and/or air condition large enclosed areas, such as stores, warehouses, factories and offices, with a number of independent conditioning units dispersed through the enclosed area. Each of the units is equipped with its own thermostatic controls for maintaining selected conditions in the area served by that unit. Typically, the controls include a "setback" feature wherein the temperature to be maintained is lowered (or raised if cooling is required) during hours when the space is not occupied in order to conserve energy. An example of such a setback control system is disclosed in U.S. Pat. No. 2,276,058,
Attempts have been made to control the multiple conditioner units in large enclosed areas with a central digital computer control in order to reduce the cost of operation, however, they have typically resulted in uncomfortable or erratic conditions for the occupants primarily because parameters such as kilowatt hours of electricity or demand have been used as the primary control parameter.
It is also known to stagger the opening of control valves in a zone temperature control system as suggested by U.S. Pat. No. 2,268,761, in attempt to even demand. However, under that system, although the valves are opened at three minutes intervals, there is no requirement that any valves be turned off when others are turned on and therefore all of the valves may be open at once.
Likewise, U.S. Pat. No. 2,958,755 discloses a control circuit which turns on a number of strip heaters one at a time at spaced intervals to eliminate a sudden demand for electrical energy; however, ultimately all of the heaters are on at the same time. Since, unlike equipment incorporating electric motors, there are no peaks created by turn on of resistance type heaters, there is no reduction in peak power drawn by this system but only a reduction in the rise time of the demand.
Often an installation will utilize a number of refrigeration units. For instance, a typical supermarket will have several frozen food cases, dairy cases, meat cases, vegetable cases, ice cream boxes and meat lockers each having one or more refrigeration units. Typically, these units have compressors, pumps, circulating fans and defrosters which draw substantial amounts of electric power and contribute significantly to the cost of doing business, hence it is important that the equipment be operated as efficiently as possible. A common problem, especially in the units open to ambient air, is the buildup of ice on the evaporator coils. The ice acts as an insulator causing the unit to run continuously in trying to lower the temperature to the set point temperature whether that temperature is controlled by a thermostat or by refrigerant pressure controls as in many commercial installations. This buildup of ice necessitates periodic defrosting of the evaporator coils such as by hot gas or by resistance heating. In many freezer installations, three or four ninety minute defrosting cycles per day are required.
It is a primary object of the present invention to operate a multiple unit temperature conditioning system at increased efficiency while still providing consistent temperature conditions.
It is also an object of the invention to provide such a system with means for signaling conditions which reduce the efficiency of the system.
It is another major objective of the invention to provide such a system which reduces the demand for energy and reduces energy consumption.